1. Field of the Invention
The present invention relates generally to a printed circuit board (PCB) in which a condenser laminate or a capacitor is embedded. More specifically, the present invention relates to a PCB including embedded polymer condenser laminates, which is capable of manifesting higher capacitance density per unit area than conventional PCBs including embedded capacitors, so that capacitors having various capacitance values, for example, multilayered ceramic capacitors (MLCCs) having high capacitance, can be embedded in the PCB, instead of being mounted on the PCB; and a method of manufacturing the same.
2. Description of the Related Art
In general, capacitors store energy in the form of an electric field. When a DC voltage source is applied to a capacitor, the capacitor is charged but the current flow stops. On the other hand, if an AC voltage source is connected to a capacitor, the current flows through the capacitor depending on the frequency of the applied AC signal and the value of the capacitor while the capacitor is charged and discharged.
Thus, the capacitor having the above properties acts as a passive component essential for use in a variety of purposes, for example, coupling and decoupling, filters, impedance matching, charge pumps and demodulation in electric and electronic circuits, such as digital circuits, analog circuits and high frequency circuits. Further, the capacitors, which are manufactured in various forms, such as chips or discs, have been used in the state of being mounted on PCBs.
However, miniaturization and complication of the electronic devices lead to reducing the areas for mounting the passive components on the PCB. Also, while frequencies become higher in accordance with high speed electronic devices, parasitic impedance is generated by the conductor, solder, etc., between the passive component and the IC, thus causing several problems. To solve the problems, various attempts have been made to embed the capacitor in the PCB, mainly led by the manufacturers of PCBs and electric and electronic components.
Although discrete chip resistors or discrete chip capacitors have long since been mounted on PCBs, PCBs including embedded passive components such as resistors or capacitors have only been recently developed.
In techniques of manufacturing PCBs including embedded passive components, passive components such as resistors or capacitors are provided in PCBs using novel materials and processes, to substitute for conventional chip resistors and chip capacitors. That is, the PCB including embedded passive components means that the passive component, for example, capacitor, is embedded in the inner layer of the PCB. Regardless of the size of the PCB itself, if the capacitor as the passive component is incorporated in the PCB, this is called an ‘embedded capacitor’. Such a substrate is referred to as an embedded capacitor PCB. The major characteristic of the embedded capacitor PCB is that the capacitor is intrinsically provided in the PCB without the need to mount the capacitor on the PCB.
FIGS. 1a to 1e show a conventional process of manufacturing a PCB including embedded polymer film type capacitors, in which a polymer capacitor paste is applied on a substrate and then hot dried (or cured), to realize the PCB including the embedded polymer film type capacitor.
In a first step, a copper foil layer of an inner layer 41 of the PCB comprising a FR-4 reinforced base sheet 42 is coated with a dry film, followed by being exposed and developed. Then, the copper foil layer is etched to form anodic copper foils 44a and 44b, cathodic copper foils 43a and 43b, and spaces therebetween (FIG. 1a).
In a second step, capacitor pastes 45a and 45b composed of a polymer that contains ceramic powder having a high dielectric constant are applied on the cathodic copper foils 43a and 43b by a screen printing process, and then dried or cured (FIG. 1b). Herein, the screen printing is performed by applying a media such as ink on a stencil screen using a squeeze, thereby transferring a pattern to a substrate.
At this step, the spaces between the anodic copper foils 44a and 44b and the cathodic copper foils 43a and 43b are covered with the capacitor pastes 45a and 45b. 
In a third step, a conductive paste including silver or copper is formed into anodes 46a and 46b using a screen printing process, and dried or cured (FIG. 1c).
In a fourth step, the capacitor layer subjected to first to third steps of the inner layer 41 of the PCB is inserted between insulating layers 47a and 47b, followed by being laminated (FIG. 1d).
In a fifth step, a through hole and laser blind via holes 49a and 49b are formed through the laminate, whereby the capacitor present in the inner layer of the PCB is connected to positive terminals 51a and 51b and negative terminals 50a and 50b of IC chips 52a and 52b mounted outside the PCB, thus acting as an embedded capacitor (FIG. 1e).
Likewise, there are disclosed methods of manufacturing the embedded discrete type capacitor by coating the PCB with a ceramic filled photosensitive resin, which have been patented by Motorola Co. Ltd., USA. The above method comprises applying the photosensitive resin containing ceramic powder on the substrate, laminating copper foil on the resin layer to form upper electrodes and lower electrodes, forming a circuit pattern, and then etching the photosensitive resin, to realize the discrete capacitor.
Further, there are proposed methods of fabricating the embedded capacitor by separately including a dielectric layer having capacitance properties in the inner layer of the PCB, so as to be used instead of a decoupling capacitor mounted on the PCB, which have been patented by Sanmina Co. Ltd., USA. In this method, the dielectric layer comprising power electrodes and ground electrodes is incorporated in the inner layer of the PCB, to obtain a power distributed decoupling capacitor.
Various processes are under study to achieve the above techniques, in which methods of embodying each process vary.
In this regard, U.S. Pat. No. 5,079,069 granted to Howard et al. discloses a capacitor laminate for use in capacitive printed circuit boards and methods of manufacture, in which the concept of ‘borrowed capacitor’is used, to manufacture the PCB comprising the laminated capacitor as a structurally rigid assembly formed of sheets of conductive material and an intermediate sheet of dielectric material, which is in operative connection with a large number of devices.
Also, U.S. Pat. No. 5,010,641 granted to Sisler et al. discloses a method of making a multilayer printed circuit board to eliminate the need for the by-pass capacitor by providing one or more fully cured power-ground plane sandwich components which are laminated together with other partially cured component layers of the board and circuit pattern-formed components.
In the embedded capacitor PCBs according to the conventional techniques, the chip type capacitor is embedded in the portion of the PCB, or the singular sheet type dielectric layer is inserted between the layers constituting the PCB.
However, the embedded capacitor PCBs manufactured by the above techniques have capacitance density of 0.5 to 3 nF per inch, which is an insufficient value to completely embed the capacitor in the PCB. Moreover, limitations are imposed on reduction of the mounting area of the passive component.
Therefore, there are required novel techniques of embedding high density capacitors, such as MLCCs, which are mounted on the PCB but not embedded therein at present, in the PCBs, by realizing higher capacitance density per unit area than conventional embedded capacitor PCBs.